This invention relates to dual systems for maintaining a database of records and in particular to a database system for maintaining accounting records in respect of a plurality of postage meters which have a facility for communication with a central computer system.
Communication between the central computer system and the meters is utilised for the purposes of recrediting of the meters from the central computer system and for transmitting accounting data from the meters to the central computer system. The central computer system maintains a database comprising records of accounting data in respect of each of the postage meters.
In known postage meters, the meter includes electronic accounting circuits for carrying out accounting functions in relation to usage of the postage meter in carrying out franking operations in which mail items are franked with desired values of postage charge. In so-called prepayment meters a value of credit is registered in a descending credit register and during each franking operation in which a mail item is franked, the accounting circuits carry out accounting functions in which inter alia the credit value registered in the credit register is decremented by the amount of the postage charge with which the mail item is franked. Periodically, when the value of credit registered in the credit register has been decremented to a low value, the postage meter is caused to communicate with a central computer system in a recrediting operation to enable further credit to be entered into the credit register of the postage meter to enable the meter to be used to perform further franking operations. As a part of the re-crediting operation, accounting data including the current value of credit in the descending credit register and accounting data registered in other registers of the meter is read from the registers of the meter and input to the central computer system.
Previously it has been necessary for the meter to be physically transported to the postal authority to enable postal authority personnel to read accounting data registered in registers of the meter, to reset the credit register with a new value of credit and to enter the data read from the registers and the added value of credit into a postal authority computer system. However it is preferred to carry out the register reading and recrediting of the meter remotely at the location at which the meter is used which avoids the need for physical transportation of the meter to the postal authority. The remote recrediting may be performed by use of a transportable memory module in a system which is the subject of our UK Patent 2173738. A further method of remote recrediting uses telephonic communication between the meters and the central computer. Systems for remote recrediting by telephone are the subject of our European Patent Application No. 89313220.9 (publication No. 0376573) and UK patent application No. 9127477.9 now abandoned.
It will be appreciated that it is a requirement that the risk of loss of data from the database maintained by the central computer system is extremely low. Accordingly during the course of a transaction, e.g. a recrediting operation, the accounting record for the transaction is written to a main database maintained in a non-volatile storage device, e.g. a hard disc storage device, and additionally an archive record is stored in an archive database which contains substantially the same records as the main database and is maintained independently in another memory device such as a second hard disc storage device. Also it is normal practice to make a back-up copy of the main database by regularly copying the main database to a tape streamer storage device, such back-up copy usually being made on a daily basis. If the main data base fails at any time it can be reproduced from the archives plus the last back-up. Because the main database, the archive database and the copy on tape are recorded on independent storage devices the risk of loss of data is very low.
Central computer systems provided for effecting transactions with remote devices such as postage meters are usually required to provide uninterrupted continuous service. Accordingly in order to prevent failure of the system due to any single failure it is common to duplicate the components of the system. Thus instead of a single computer, which may be a so-called personal computer, two computers are provided. Normally a first one of the computers is arranged to operate as a master or primary computer and carries out all transactions but in the event of failure of that computer, the other computer, which up to that time has been quiescent or operated in a reserve secondary capacity is caused to carry out the transactions. A difficulty then arises to ensure that the data in a database maintained by the secondary computer is up to date and corresponds to the data in the database which at the time of failure has been maintained by the first or primary computer.
If the data recorded on back-up tape is utilised to retrieve data into the database of the second computer, manual intervention is usually required and there is a break of undefined length in operation of the system while the retrieval process is effected. This is commonly known as cold standby.
An alternative is to operate the second computer in a manner which maintains the database of the second computer up to date with the data in the main database of the first computer at all times. Accordingly in the event of a failure of the first computer, the second computer is ready at any instant to take over from the first computer and to continue maintenance of the database of the second computer.